1. Technical Field
The present invention relates to a method and apparatus for producing a strip-like crystal such as silicon crystal by continuous pulling of the growing crystal, which is suitable for use, for example in a solar battery.
2. Background Art
Crystal growing of materials such as silicon for semiconductor devices is performed as follows. First, a seed crystal is dipped in a molten liquid of the required substance held in an overcooled state in a crucible. Second, a continuous crystal can be grown on the seed crystal from the melt in the crucible by slowly pulling the seed crystal upwards. This method is known as the Czochralski (CZ) method, and uses the apparatus shown in FIG. 6. A seed crystal 12 suspended on a wire hanging from a pulling device 11 at the top section of the furnace is dipped in the melt 14 held in a crucible 13 to commence the crystal growing process as shown in FIG. 6. The wire 15 is wound up slowly by the pulling device 11 so that a crystal 16 may grow on the seed crystal 12. When the seed crystal reaches the upper end of the pulling device 11, the crystal growing process for the growing crystal 16 is terminated. Here, reference numerical 17 shows a heater for controlling the temperature of the melt 14 inside the crucible 13. Also, reference numerical 18 shows an insulator surrounding the heater 17. At this stage, the process is returned to the initial step to restart the crystal growth. However, during the time interval between the termination of one crystal growth and the startup of the next crystal growth, many preparatory steps have to be taken such as re-adjustment of the melt temperature, detaching the grown crystal and attaching a new seed crystal. Thus, it results in a labor-intensive and low-productivity process.
For growing ribbon-shaped crystals of about 0.1 mm thickness, the crystal can be pulled continuously using a reel-type device such as the one shown in FIG. 7, because such thin crystals are flexible. In this type of apparatus, a seed crystal 22 is attached to an end of flexible tape 21 while the opposite side of the tape 21 is attached to a pulley of a winding reel. The crystal growing process by rotating the pulley 23 is as follows. It starts to lower the seed crystal 22 by rotating the pulley 23 to contact the melt 14. Then, the pulley 23 is rotated in the opposite direction to pull the growing crystal upwards. The seed crystal 22 and the grown crystal on the seed crystal can be continuously wound on the pulley 23. In principle, this method is able to produce crystals of an extended length, and it is suitable for growing ribbon-shaped flexible crystals.
However, because there is some temperature gradient in the melt 14 held in the crucible 13, it is necessary for the seed crystal to be dipped precisely in the center of the crucible 13 when starting to grow a crystal on the seed crystal. Also, after commencing crystal pulling, it is necessary to continually maintain the growing crystal in the precise location. However, in an apparatus such as the one shown in FIG. 7, because of the flexibility of the tape 21, the seed crystal 22 can undulate while it is being lowered, thereby making it difficult for the seed crystal 22 to contact the required central location of the melt. The initiation process of crystal growing operation is, therefore, a laborious process, and yet such effort may still result in failure, or even after a successful initiation of a tape crystal growing operation, the pulling process may still have to be discontinued because the growing tape crystal may oscillate so as to interrupt the growth process.
It is an object of the present invention to provide an apparatus and a method to enable a seed crystal to be directed to the precise location of a melt for growing a ribbon-shaped crystal. After the crystal has commenced growing, the ribbon-shaped crystal is continuously pulled up so as to produce a longitudinally extending crystal using a continuous pulling device.
To achieve the object of the present invention, there is provided a method for producing a ribbon-shaped crystal comprising: growing a ribbon-shaped crystal on a seed crystal using a linear pulling device for pulling the seed crystal and a crystal growing at the end of the seed crystal in a vertical direction, and continuing to pull the ribbon-shaped crystal by using a continuous pulling device having a continuous pulling mechanism.
According to the present method, at the beginning of the crystal growing process to start growing a crystal on the seed crystal, the linear pulling device is used to lowered the seed crystal in a precise desired location of the melt and to start to pull the growing crystal in the vertical direction. This approach enables a ribbon-shaped crystal of a desirable dimensions to grow in a stable and reliable manner. After a sufficient length/height of the ribbon-shaped crystal has been pulled up using the linear pulling device, the continuous pulling device is used to pull the growing crystal continuously so as to produce a continuous strip of ribbon-shaped crystal. Therefore, the method, in principle, is capable of producing a longitudinally extending crystal of a considerable length.
Also, there is provided an apparatus for continuous production of a ribbon-shaped crystal comprising: a linear pulling device for linear vertical pulling of a seed crystal and a ribbon-shaped crystal grown on the seed crystal; a continuous pulling device for pulling the crystal continuously by clamping a portion of the ribbon-shaped crystal; a switching device for the changing pulling device from the linear pulling device to the continuous pulling device before the seed crystal has passed through the continuous pulling device; and a crystal cutting device for severing the seed crystal from the grown ribbon-shaped crystal.
It is preferable that the linear pulling device is provided with a position adjusting mechanism for adjusting the seed dipping location of the seed crystal. It is also preferable that the continuous pulling device is provided with a device for adjusting the position of the continuous pulling device so as to properly initiate the growth of a crystal on the seed crystal. It is also preferable that a crystal cutter is provided to sever the ribbon-shaped crystal grown by continuous pulling into sections of a suitable length.